In recent years, the shortage of transmission line bands is coming to the surface along with the increase in the transmission speed. A problem caused by the shortage of transmission line bands is intersymbol interference. The intersymbol interference is the interference between adjacent signals each representing a bit value “0” or “1”. Since the waveform of a signal degrades due to the intersymbol interference, it is difficult to determine the bit value from the input signal on the receiver side.
A method for solving the degradation in the waveform of the input signal is waveform equalization. Waveform equalization is a technique of intentionally changing part of a signal waveform in accordance with degradation in the signal waveform caused by intersymbol interference.
One of the waveform equalization techniques is decision-feedback equalization that performs processing only on the receiver side. The decision-feedback equalization is disclosed in, for example, non-patent literature 1. In the decision-feedback equalization, a bit value represented by an input signal is determined from the waveform of the input signal. Based on the determination result, the influence of degradation in the waveform of the already received input signal is removed from the next signal input to the receiver.
FIG. 15 shows an example of an input signal degraded by intersymbol interference. Referring to FIG. 15, C0 represents a main tap that is a bit to be determined; C−1, a precursor that is the waveform distortion of the bit immediately before the main tap C0; C1, a first post-cursor that is the waveform distortion of the bit immediately after the main tap C0; C2, a second post-cursor; and C3, a third post-cursor. In the decision-feedback equalization, the intersymbol interference from the first post-cursor C1 can be removed.
In the decision-feedback equalization, the determination result is fed back. Hence, feedback to the bit next to the determined bit, that is, the first post-cursor is the fastest. The transmission rate is rate-limited by the delay time of the feedback. To avoid the rate-limitation of the transmission rate, speculative decision-feedback equalization has been proposed. The speculative decision-feedback equalization is disclosed in, for example, non-patent literature 2. In the speculative decision-feedback equalization, equalization is executed in advance for both preceding bits having values “0” and “1”. After determination of the bit value, the correct execution result is selected and output. For this reason, the fastest feedback to the first post-cursor is unnecessary. This allows to increase the time constraint of feedback and achieve a higher transmission rate.